1. Field of the Invention
The present invention relates to a portable, hand-held and nondestructive corrosion sensing device for detecting the early stages of metal and coating degradation and electrochemical corrosion. More specifically, the present invention relates to a portable corrosion sensor which is utilized under field (actual, environmental or in situ) conditions in detecting coating degradation and electrochemical corrosion of both small and large coated (painted) and uncoated metal structures, thereby permitting detection of coating and metal degradation and electrochemical corrosion well before serious deterioration of the material or structure has occurred.
2. Prior Art
A major goal in the electrochemical field has long been to create a sensor which could be utilized in field or service conditions to detect corrosion and adhesion on metal structures of any size before significant degradation has occurred. Evaluation of materials and coatings and the determination or prediction of corrosion performance of both painted and uncoated metal structures or specimens under ambient field or service conditions has traditionally involved visual comparisons which are subjective and require blistering, rusting, or other advanced stages of degradation. The use of laboratory techniques, such as electrochemical impedance spectroscopy (EIS, also known as AC impedance), which has been used to understand and predict corrosion performance during immersion exposures in different electrolytes was limited to small structures or witness specimens that could be immersed, small sections of material cut from large structures, or attachment to the structure of a clamp-on liquid cell in which a liquid or semi-liquid electrolyte and remote counter and reference electrodes were contained.
Inspection of a large structure using conventional EIS methodologies required complete immersion or use of a clamp-on cell. Such cells would be filled with a liquid or semi-liquid electrolyte (e.g., Kihira et al., U.S. Pat. No. 4,806,849; and Kazami et al., U.S. Pat. No. 4,861,453) or a spongy medium impregnated with a liquid electrolyte (e.g., Kondou et al., U.S. Pat. No. 5,221,893) with remote electrodes immersed in the electrolyte or in intimate contact with the electrolyte-impregnated sponge. These cells required an accessible, flat, smooth, and horizontal area. The set-up was considered to be time consuming and had to be performed for each measurement. Corrosion was detected only directly under the cell and use of the cell actually caused artifactual damage to the coating in many instances because of exposure to the electrolyte during measurement.
Davis et al., U.S. Pat. No. 5,859,537, recently taught a painted electrode sensor which eliminates many of the problems discussed above. The actual structure is being inspected without exposure to an extrinsic electrolyte. Measurements are possible under most natural or accelerated conditions and material and coating degradation are detectable from the very early stages. However, the Davis et al., sensor requires an electrode to be permanently painted onto the structure and is time-consuming, because of all of the fabrication steps which must be completed. It is not suitable for structures in which appearance or aerodynamics preclude an attached sensor. The sensor can induce artifactual damage in a small class of materials, primarily porous coatings.
Presently, there is no portable, hand-held corrosion sensing device for early detection of electrochemical corrosion, metal and coating degradation which can evaluate degradation on structures or material of any size, under in situ or actual conditions, as well as under aggressive corrosive conditions, and which requires no permanent attachment.